Alzheimer's disease (abbreviated as AD) is a neurodegenerative disease characterized by dementia. Ad insidiously develops in old age and gradually progresses to dementia. AD is pathologically characterized by large amount of senile plaques and neurofibrillary tangles (abbreviated as NFT) observed in the cerebral cortex and hippocampus, and by cerebral atrophy due to deciduation of neuron.
Senile plaque has, as a main component, an amyloid fiber comprising of an extracellularly sedimented amyloid β-peptide (abbreviated as Aβ). The appearance of AB is extremely specific for AD and aging. Senile plaque is a pathological change which is seen at the earliest stage of the disease's progression. Therefore, an understanding of the mechanism of formation and sedimentation of Aβ has a very important significance in elaborating the morbid state of AD.
Aβ includes, two main molecular species; Aβ 40 that ends at the 40th amino acid residue Val; and Aβ 42 which is larger by two residues (Ile, Ala). Among the Aβ secreted from cells, about 90% is Aβ 40, and 10% is Aβ 42. However, Aβ 42 exhibits aggregating property which is far greater than that of Aβ 40. Aggregated Aβ 42 entangles with Aβ 40 to form amyloids. In addition, because Aβ 40 accumulated during the initial stage of plaque formation, it has been suggested that the accumulation of Aβ 40 was strongly associated with the development of AD.
Aβ is produced by cleavage of an amyloid precursor protein (abbreviated as APP) by two kinds of proteases. APP is a type I transmembrane protein consisting of about 700 residues, with Aβ contained in its extracellular region at a transmembrane region. A protease that is involved in the production of Aβ from APP is called “secretase”. A secretase that cuts (cleaves) the amino terminal of Aβ is a “β secretase”; a secretase that cuts the carboxyl terminal is a “γ secretase”; and a secretase that cuts the carboxyl terminal of the 16th Lys in the interior of Aβ is an “α secretase”. In an amyloidogenic pathway, APP is cut with a β secretase to produce a secretion-type APP β (sAPP β) and a carboxyl terminal fragment called β-stub. A γ secretase cuts this fragment to produce Aβ and a γ-stub. In a non-amyloidogenic pathway, an a secretase cuts APP to produce secretion-type APPα (sAPPα) and a carboxyl terminal fragment called α-stub. A γ secretase cuts this fragment to produce p3 and γ-stub. These two pathways both exist in the normal state. The majority of Aβ that is produced as a physiological molecule is secreted extracellularly.
One of the current mainstream strategies is to target secretase as a point of action for preventing or treating AD. Among them, particular attensions are paid for β and γ secretases since direct effects can be readly observed. In recent years, pharmaceutical enterprises and venture companies vigorously progress in the development of secretase inhibitors.
In 1999, it has been reported by some laboratories that β secretase is a β-site APP cleaving enzyme called BACE1 (also referred to as BACE, Asp2, or memapsin2).
Since BACE1 is an aspartic acid protease, based on the substrate transition state concept established by the study on the same class of protease inhibitor such as renin and HIV, various β secretase inhibitors have been reported in recent years. Swedish variant-type APP readily produces Aβ. A peptide-based inhibitor mimics Swedish APP by forming a substrate transition state analogue at a β secretage cleaving site. Tung et al. determined the IC50 value of 30 nM in a 14 residue peptide (P10-P4′ statV) in which a hydroxyethylcarbonylisostere (statine) structure was introduced into a substrate cleaving site of β secretase. They reported that other substrate transition state analogues (AHPPA, ACHPA) can also inhibit β secretase (for example, see Nature, 402, 537-540 (1999), J. Med. Chem., 45, 259-262 (2002)). Ghosh et al. searched various compounds containing hydroxyethyleneisostere, and discovered that OM 99-2 had strong inhibitory activity (Ki=1.6 nM) (for example, see WO 01/00665, J. Am. Chem. Soc. 122, 3522-3523 (2000). Based on X-ray crystal structural analysis of an enzyme-OM99-2 complex (for example, see Science 290, 150-153 (2000)), they synthesized derivatives, and found a smaller inhibitor having equipotent activity (for example, see J. Med. Chem., 44, 2865-2868 (2001)).
However, in considering an acting site of a β-secretase inhibitor, a drug needs to pass through the blood-brain barrier. There are many points to be considered such as a molecular size and hydrophilic/hydrophobic balance. In addition, since substrate specificity greatly differs from previous aspartic acid proteases (for example, see Biochemistry, 40, 10001-10006 (2001)), it is expected that the design of inhibitors is very difficult. Furthermore, in order to be physiologically active, it is necessary for the inhibitor to be able to pass through the blood-brain barrier. For this reason, compounds in which the molecular weight is relatively small, and stability in blood is excellent are desired. Designing compounds with such features are thought to be difficult.